1. Field of the Invention
This invention pertains broadly to the field of plasma generating devices such as those used in the semiconductor processing industry. More particularly, but without limitation thereto, the invention relates to the delivery of ionizing energy within a semiconductor processing system so that a plasma is induced at a desired location within the system.
2. Description of the Prior Art
The process of forming plasmas is well known to the semiconductor industry. The plasmas facilitate the deposition of selected materials upon semiconductor substrates as well as serve to etch or remove selected materials from surfaces. In prior art systems, plasmas are typically generated by way of radio frequency (RF) and microwave discharges.
In typical radio frequency discharge devices, a plasma is formed by coupling RF energy to a gas via electrodes, coils or plates. In common microwave discharge devices, microwave energy is coupled to a gas by way of a hollow waveguide and a resonant cavity.
In microwave discharge plasma generating devices, control of plasma location, intensity and energy has been limited. This has resulted in non-uniform plasma processing affecting both the uniformity and yield of electronic devices processed.
In these devices, microwaves are generated in a section of the processing equipment that is remote from the location of the substrate to be processed. In order to transport the microwaves to the substrate, a hollow metal waveguide is coupled between the source of microwaves and the reaction chamber in which the substrate is contained.
In some microwave discharge devices, plungers are used to dimensionally tune a reaction chamber to resonance so that a plasma becomes positioned in an area in which the substrate lies. The size of the resulting plasma is limited and the plasma is not entirely uniform.
In semiconductor processes requiring uniform heating it is often desirable that a uniform plasma coincide with an area or zone of uniform heating. Long tubular reaction vessels with heating elements surrounding the vessel are often used to form the zone of uniform heating.
With such a set-up, a central heating zone is created within the vessel with separate heating zones on either side of the vessel. The central area of uniform heating, otherwise known as a "hot zone", usually lies equidistant between the ends of the chamber. Because of the often high temperatures involved and because of mechanical and power supply constraints, the source of microwaves, including a signal generator, driver, amplifier, etc., generally must be located a safe distance from the heated reaction chamber.
In cases in which heating elements surround the length of the chamber, the microwaves must be introduced at one end or the other of the chamber. As typical heated chambers run as long as six feet, a minimum of three feet will exist between the central hot zone of the chamber and the entrance of the microwave energy.
If microwave energy could be carried directly to the reaction chamber's hot zone, a plasma could be formed within the uniformly heated area of the chamber.
If these microwaves were transported to the hot zone by the traditional hollow metal waveguide, contamination of the plasma process would occur as the metal waveguide would eventually degrade in the elevated temperatures of the heated reaction chamber.